Abstract Multiple Sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) characterized by demyelination and axonal loss. Demyelinating lesions are defined by cellular infiltrates composed predominantly of T lymphocytes and two distinct types of myeloid cells, namely CNS infiltrating bone marrow- derived macrophages (BMDM) and resident microglia. Understanding their interplay is essential to MS pathogenesis, as T cell-produced cytokines and chemokines promote myeloid cell populations to produce toxic factors and strip myelinated axons, culminating in tissue damage. However, how T cells instruct myeloid cells, as well as the relative contributions of BMDM and microglia to tissue damage and disability, are unclear. Specifically how Th1 and Th17 cells, both found in MS patients, affect BMDM and/or microglia to express toxic functions, as well as strip and phagocyte myelin, are still poorly understood. The overall goal of this proposal is to define in vivo mechanisms that regulate microglia and BMDM to actively participate in the demyelinating process. Using a unique murine virus encephalomyelitis model in which microglia mediate demyelination in the absence of BMDM and Th17 response, this proposal will define how distinct T cell functions specifically promote microglia-mediated demyelination. This model provides a unique tool to dissect how Th1 versus Th17 cells regulate microglia effector functions during demyelination. Based on our preliminary data, we hypothesize that strict Th1 conditions drive microglia to mediate demyelination, while Th17 response can modify this effect by altering microglial pathogenic functions and promoting BMDM-mediated demyelination. Aim 1 will test how microglial responsiveness to T cell-derived IFN-? regulates their demyelinating function independent of BMDM. More specifically, this aim will define the contribution of microglia oxidative burst to myelin damage, as well as determine whether TREM-2 modulation of microglial phagocytic functions can contribute to demyelination. Aim 2 will reveal how Th17 cells alters microglia effector functions and myelin damage, in the presence or absence of BMDM, and whether this effect is directly dependent upon IL-17 and/or GM-CSF secretion. Gene array analysis will also characterize phenotypic markers associated with the pathogenic versus protective functions of microglia in a distinct inflammatory environment. By revealing how microglia respond to prominent T cell cytokines, this proposal will provide new insights into the direct contribution of microglia to lesion formation in MS patients, potentially leading to new therapeutic targets.